The dark specks swirling around in big water-filled tanks at the Columbus Center hardly look like fish, much less the kings of the ocean.

But from these tiny beginnings, a team of Maryland scientists hopes to unlock the secrets of "farming" Atlantic bluefin tuna, one of the most prized fish on the planet — and one of the most threatened.

"For me, it's the Holy Grail," said Yonathan Zohar, a professor of marine biotechnology with the University of Maryland, Baltimore County and head of the aquaculture research center at the Institute for Marine and Environmental Technology at the Inner Harbor.

He and his colleagues have been trying this summer to nurture newly hatched bluefin tuna from microscopic, translucent larvae to fingerlings recognizable as miniature fish.

It is perhaps the most ambitious effort yet for Zohar, who raised everything from blue crabs and striped bass to amberjack, cobia, sea bream and salmon in large tanks on the ground floor of the Columbus Center. As he and his team struggle to raise the bluefin tuna larvae, he hopes this summer's labors might one day help replenish the wild stocks of an iconic fish.

The Atlantic bluefin tuna is the largest of all the tuna, growing to nearly 10 feet and up to 1,600 pounds over a life span of up to 30 years. The tuna range from the North Atlantic to the Mediterranean Sea, sprinting through the water at 45 mph or more as they chase down and devour fish, crabs, octopus, jellyfish and other marine creatures.

Perhaps because of their size and place at the top of the marine food chain, bluefin tuna have been much sought-after, especially as sushi. A single 489-pound Pacific bluefin garnered $1.76 million in an auction in Japan last year. Such finny bounty has attracted outsized fishing pressure.

Bluefin are truly "superfish," said Dave Bard, spokesman for The Pew Charitable Trusts, "but they're no match for indiscriminate fishing gear intended primarily for yellowfin tuna and swordfish. Their population has declined by nearly two-thirds from its 1970 level, he said.

The National Oceanic and Atmospheric Administration counts Atlantic bluefin tuna among its "species of concern," not quite endangered but in need of protection. U.S. and international authorities have adopted fishing restrictions, but some question how effective they are at halting the decline, much less rebuilding the population.

With the commercial catch limited, efforts have been launched in Japan, Australia and Europe to produce bluefin tuna in captivity. Young fish have been successfully reared in pens, but the challenge now is to "close the life cycle," as Zohar puts it, and raise them from eggs to fingerlings.

Little is known about how to do that, however. So the Columbus Center team is tending its clear plastic fish tanks round-the-clock, trying to find the right mix of food and water conditions to get their charges to develop into fingerlings. Mortality has been high, but the survivors of eggs spawned last month are growing rapidly.

Zohar's team is getting fertilized bluefin eggs from a tuna "ranch" on the Croatian coast of the Adriatic Sea. Tuna rounded up from the wild are fattened for market at the ranch there run by Kali Tuna before they are sold, primarily to Japan. But the company agreed a couple of years ago to keep two large pens of bluefin tuna to breed and supply Zohar's aquaculture project.

The pens are kept under close watch, and as soon as eggs appear on the water, they are skimmed out and rushed to Baltimore, Zohar said. It's a 36-hour odyssey, starting with a boat ride to the Croatian mainland, where they're loaded onto a plane in Zagreb, the capital. The eggs then hopscotch across Europe by air before flying over the Atlantic to New York.

There, Zohar has a driver waiting to pick up the shipment as soon as it clears customs and rush it to the Columbus Center. It's a race to get the eggs into Zohar's tanks because if they hatch en route, they're lost. More than one shipment has fallen victim to missed connections and other problems.

After poor reproduction last year limited his research, Zohar said, he's gotten more eggs this summer, but not as many as he had hoped.

"So far, so good," he said. "Every day is a victory. ... The bottleneck is the first two months. If we get past that, we'll open a bottle of Champagne."

The budget for the project has been around $450,000, which Zohar calls "very modest for such a global and a complex project." Unable to get federal funding, he has pulled together support from a foundation and from private companies, including BP.

Zohar said he hopes the project will eventually ease the pressure on wild stocks, producing marketable 30- or 40-pound fish in large recirculating tanks. Ideally, he'd like to see hatcheries developed that could help restock the oceans with bluefin.

But conservationists have mixed feelings about fish farming, especially when it involves big oceangoing fish such as bluefin tuna.

"We are generally concerned about the propagation of top-order carnivores as a means to meet seafood demands‎ when it will cause a burden on wild fish either fed whole or incorporated into feed," said Aaron McNevin, aquaculture director at the World Wildlife Fund.

In the same way that cattle consume up to 7 pounds of feed for every pound of meat they produce, tuna raised in "ranches" are fed several times their weight in other fish.

McNevin said his group supports the kind of "closed-system" aquaculture developed at the Columbus Center. But if by raising bluefin tuna in captivity "we are setting the stage for large demands on forage fish, we are not providing any relief to the oceans," he cautioned.

Zohar acknowledged that the bluefin's diet is an issue that must be resolved to raise them sustainably. But he said researchers have to figure out how to successfully raise fingerlings from eggs before tinkering with what they eat.

Scientists at the Columbus Center have worked to develop plant-based, more environmentally sustainable feed for carnivorous fish.

Michael Rubino, director of aquaculture in the NOAA's fisheries service, said he's following Zohar's work with bluefin tuna, even though he considers it too speculative at this point to invest limited federal dollars.

"The challenge is, growing tuna to 2 or 4 years old is difficult, expensive and still pretty uncertain," Rubino said.

Scientists have made more progress in cultivating yellowfin and blackfin tuna, he said, which has greater commercial potential because the two species are more widely sold in sushi bars and restaurants.

Still, Rubino said, the benefits of the work underway at the Columbus Center go beyond bluefin tuna to showing the way to more sustainable fish farming in general. Zohar has developed a closed aquaculture system, which addresses concerns about water pollution in fish farms by filtering out fish waste from the tanks and recycling the water.

"We will work on it one step at a time," Zohar said. "First, we have to open this bottleneck of closing the life cycle. By summer's end, "If I have 100 2-inch fish for this year, it will be mission accomplished."